Field of Invention
The present invention relates to a technical field of photoelectric detection, and more particularly to a combined leg structure of a micro bridge unit of a focal plane array.
Description of Related Arts
Because of the advantages of: (a) a low l/f noise, (b) a high temperature coefficient of resistance (TCR), and (c) a good process compatibility with the micro-electro-mechanical system (MEMS), the vanadium oxide film, as the thermal resistance sensitive film, is widely applied for the preparation of the microbolometer uncooled focal plane array and the corresponding uncooled detector with the excellent detection performance. The vanadium oxide, as the sensitive material, is also applied in the manufacture of the THz detection focal plane array for an object detection and identification in the THz band (0.1-10 THz). Moreover, due to the distinct phase transition characteristics near the room temperature, the vanadium oxide film has the wide application prospect in the fields of the temperature sensor, the gas sensor, the electrochromic device and the optical switch.
The arrayed sensitive unit structure is widely adopted by the high-performance detectors and sensors. The concrete form of the array structure depends on the application request of the device. Taking the uncooled focal plane array based on the vanadium oxide sensitive film as an example, the focal plane array generally comprises a set of the two-dimensional micro bridge unit arrays, wherein each micro bridge unit comprises a sensitive area and two bridge legs for supporting. The bridge legs are not only for mechanically supporting the sensitive area, but also is the extraction electrode of the sensitive area and the important thermal conduction pathway. The radiation of the detected object is projected to the sensitive unit and then absorbed, increasing the temperature of the sensitive area; meanwhile, the heat flows to the surrounding environment through the thermal conduction, the thermal convection and the thermal radiation. The flow of the heat decreases the temperature rise range of the sensitive area, so as to decrease the response of the detector. Thus, in order to achieve the high detection performance, the array is generally sealed in the vacuumed shell and each micro bridge unit of the array is isolated from each other. Accordingly, the thermal conduction through the bridge legs to the substrate becomes the main heat loss manner of the micro bridge unit. The temperature rise of the sensitive unit leads to the change of the resistance of the thermosensitive film. And the change of the resistance is probed by means of the high-electricity-conductivity film layer inside the bridge legs as the extraction electrode, thus the object is detected.
In order to obtain high-performance uncooled focal plane arrays, the electrode material of the micro bridge unit thereof is generally required to have the high electricity conductivity, the low thermal conductivity and the preparation method liable to get the good compatibility with the MEMS process and the integrated circuit (IC) process. Conventionally, the common electrode materials of the vanadium oxide focal plane array include NiCr films and Ti films. Because of the good oxidation resistance, the NiCr film is the most common electrode material of the vanadium oxide focal plane array. However, the NiCr film has the large elasticity modulus, which leads to the large residual stress and accordingly the warping of the array unit. Moreover, it is difficult to pattern the NiCr film through the conventional dry etching tool and process, which decreases the process compatibility of the NiCr film to a certain extent. The Ti film is liable to be patterned and has the appropriate electricity conductivity and the thermal conductivity. However, the Ti film is liable to be oxidized, which affects the process compatibility of the Ti film. Thus, it is necessary to develop an electrode material to further improve the detection performance of the vanadium oxide focal plane array.
Moreover, in the conventional focal plane array structure, the bridge leg is regarded as the main heat loss pathway of the micro bridge unit. The micro bridge unit absorbs the infrared radiation, leading to the temperature rise of sensitive area; meanwhile, the temperature difference between the sensitive area and the bridge leg further leads to the heat loss from the sensitive area to the bridge leg and the inhibition of the final value of the temperature rise of the sensitive area. In order to decrease the heat loss from the bridge surface to the bridge leg, the design of the bridge leg usually adopts the strategy of the overlong bridge leg structure or the narrow bridge leg. However, the length of the bridge leg is limited by the design requirements of the duty cycle and the mechanical stability of the micro bridge unit. The width of the conventional bridge leg reaches 0.5 μm, and the manufacture cost of the focal plane array will greatly increase if the width of the bridge leg is further decreased. It is necessary to design a bridge leg structure to decrease the heat loss of the bridge leg, which provides a new choice to realize the high-performance vanadium oxide foal plane array.